The present invention relates to a method and apparatus for image transmission wherein an original image is read to obtain an image signal which is compressed and encoded for image transmission.
A minimum transmission is set for the transmission and reception sides of a facsimile apparatus. The minimum transmission time is the time required for reading and encoding one line of an original document at the transmission side, and the time required for recording one line at the reception side. If one line image signal is transmitted within a time shorter than the minimum transmission time, then recording at the reception side cannot follow. Therefore, in a G-3 mode facsimile apparatus conforming with Recommendation T30 of CCITT, fill bits or dummy signals have been added as shown in FIG. 1A if the transmission time of encoded information for one line is shorter than the minimum transmission time.
Meanwhile, there has been proposed in U.S. patent application No. 672,783 (filed on Nov. 19, 1984) and commonly assigned herewith a transmission method shown as a fill-bit-less transmission mode wherein Fill bits are not added if the total transmission time for a plurality of lines is longer than the sum of the minimum transmission times for the same number of lines. By doing, so it is possible to reduce the amount of fill bits by A as shown in FIG. 1A to thereby shorten the transmission time.
The amount of fill bits in the Fill-bit-less mode is calculated as follows: The capacity of a reception buffer at the reception side for temporarily storing a received signal is first detected. The amount of data is accumulated each time one line of image data (i.e., data for one line, e.g., one scan line) is transmitted. When the accumulated amount of data exceeds the reception buffer capacity and if the accumulated amount of data is larger than the number of bits corresponding to the sum of the minimum transmission times for the same number of lines for the accumulated data, fill bits are not added. However, if the accumulated amount of data is smaller than the number of bits, fill bits corresponding in number to the difference therebetween are added to the one-line prior image data.
The accumulated amount of data and the reception buffer capacity have been compared heretofore as above. In this case, however, since the data in the reception buffer is being read and recorded during data transmission, an empty area is actually present within the reception buffer even if the accumulated amount of data exceeds the reception buffer capacity. That is, the reception buffer is not efficiently used.
It is desirable to efficiently use the buffer memory having a capacity as small as possible because the buffer membory capacity is directly related to the cost of the apparatus.
Apart from the above, it has become possible to transmit a large amount of information within a shorter transmission line due to recent advance in communication technology. For example, it is possible to transmit information at 9600 bits/sec over a public telephone network. Therefore, the fact is that the reading and encoding speed cannot follow the transmission speed. This will be explained in more particular by taking as an example a facsimile apparatus. The amount of one line encoded information, all white, is 31 bits for MH code, including an EOL signal, and 15 bits for MR code, including an EOL signal. Therefore, the transmission time is 3.2 msec for MH code and 1.5 msec for MR code, respectively, at 9600 bits/sec.
Whereas if a charge storage type image sensor such as a CCD is used, the time required for reading and encoding one line is 5.0 msec including a storage (readout) time of 2.7 msec, data transfer time of 2.2 msec and encoding process time of 0.1 msec. Thus, an idle time of 1.8 msec for MH code and 3.5 msec for MR code becomes present. Conventionally, a dummy signal which is called a fill bit has been transmitted during the idle time until encoding is completed. This idle time becomes serious, especially for the area where the amount of encoded signal is small, e.g., the blank portion at the top of an original. The amount of fill bits is extremely large at the top of an original so that the transmission time becomes long.
Furthermore, after completion of transmitting one page image data, the transmission side conventionally continues to send a dummy signal or fill bits until the reception side completes the recording process, as shown in FIG. 1B. After sending fill bits, an RTC (Return to Control) signal is sent which indicates that control signals different in transmission speed from that of image data is sent. Upon reception of the RTC signal, the reception side changes the high speed modem for the reception of image data to the low speed modem for the reception of control signals. After sending the RTC signal, the transmission side sends a preamble signal after the lapse of a predetermined time stipulated in the T30 Recommendation of CCITT. Subsequently, a Q signal is sent. The Q signal indicates that the next page image signal will be sent after resetting the communication protocol, the next page image signal will be sent leaving the current communication protocol as it is, or all of the image signals have been sent.
As descrbied above, conventionally, control signals are sent after a certain time lapse after the recording process at the reception side. Therefore, it takes a fairly long time for the process between pages, resulting in a long transmission time for an original document with a large number of pages.